Raw hydrocarbons are currently converted to commercially more useful materials by multi-step and/or high temperature processes, typically above 300° C. This leads to expensive reactors, extensive heat management and subsequent high capital and operating costs. In conversions developed to date, the key chemical challenge is the direct, selective conversion of C—H or CC bonds of hydrocarbons at lower temperatures to produce functional bonds such as C—OH, C═C, other C—C or other C—X bonds where X is a heteroatom. In general, present oxidation catalyst technology for C—H and CC conversion is not sufficiently selective to allow direct conversion processes due to the involvement of radical and especially free radical reaction pathways, for example Bhinde et al. (U.S. Pat. No. 5,723,697) incorporated herein by reference in its entirety. There is thus a need for new catalysts for converting the C—H bond to functionalized bonds that can be utilized for the conversion of hydrocarbons to more useful materials under milder and more selective conditions.
Efficient catalytic systems for the low temperature, selective oxidation of hydrocarbon alkanes to alcohols, X═OH, are Pt(II) and Hg(II) all operate in strongly acidic media. See for example Periana et al. (U.S. Pat. No. 5,233,113, U.S. Pat. No. 5,306,855 and US Patent Application 2003/0120125) incorporated herein by reference in their entirety. The metals Pt and Hg have been reported to catalyze the conversion of methane in concentrated sulfuric acid to methyl esters with formation of reduced oxidant. Subsequent hydrolysis of the methyl ester and reoxidation of the reduced oxidant comprised a complete system for the selective oxidation of methane to methanol.
A problem in devising a catalytic process for the partial oxidation of alkanes is the non-reactive nature of the alkane C—H bond and the difficulty in finding a catalytic substance which will promote activation of; and subsequent reaction at, one or more of the C—H bonds of the alkane reactant without also catalyzing complete oxidation of the alkane in question—e.g., methane to CO2. This threshold problem has been solved, to at least some degree, by the catalytic process described in U.S. Pat. Nos. 5,233,113, 5,306,855 and US Patent Application 2003/0120125).
A major disadvantage of the Pt(II) or Hg(II) systems in strong acid is that only ˜1M methanol could be developed before the reaction effectively stopped due to the effective drop in solvent acidity. This product inhibition leads to impractically high separation costs. The primary reason for this limitation in product concentration is that as both the methanol and water build up in the reaction product mixture, these molecules preferentially coordinate to the Hg(II) ions and inhibit catalysis. Consequently, designing catalysts that are not inhibited by water or product is one of the central challenges to developing catalysts that efficiently oxidize alkanes to alcohols.